Activation of p53 in propagating cells following DNA damage results in the initiation of either cell cycle arrest or apoptosis. Prime factors determining cell fate include cell type, context and the extent and type of DNA damage sustained. Activation can result in induction of p53 levels and posttranslational modification by phosphorylation and acetylation. Of these, we hypothesize site-specific phosphorylation is intimately involved in cellular decisions to produce arrest or apoptosis. While there are p53-independent mechanisms of accomplishing growth arrest or cell death, exposure to ionizing radiation and reactive oxygen species like hydrogen peroxide, H2O2, are very representative environmental stimuli that produce p53-dependent arrest or apoptosis. The selective transcriptional activation by "activated" p53 of specific downstream target genes appears to be dependent upon a complex set of events that have not yet been elucidated. Recent evidence suggests that modulation of p53?s interactions with transcriptional cofactors like ASPP and JMY may be involved in transcription of critical downstream target proteins like p21, bax and others in determining cellular fate. We believe that specific posttranslational modifications of p53 are important determinants for competing with a limited pool of transcriptional cofactors that can determine cell growth and death. Regarding phosphorylated p53 and growth arrest, a number of amino and carboxy-terminal sites on p53 have been examined by our group using phosphospecific antibodies in two type of wild type (wt) p53 expressing cells, IMR90 normal diploid fibroblasts and in HT1080 cells, at wt p53 fibrosarcoma line. Several amino- and carboxy terminal serine sites have been found in both cell types with some constitutive level of phosphorylation. However, only Ser-15 was significantly changed with gamma irradiation in both IMR90 and HT1080 cells. While IMR90 cells exhibit G1 growth arrest, HT1080 do not and the defect in cell cycle arrest in HT1080 tumor cells lies with the pRb tumor suppressor, rather than in altered p53 phosphorylation. Regarding H2O2-induced apoptosis we found that a kinase at a specific amino-terminal site on p53 was phosphorylated preceding apoptosis. Most recent studies show induction of recently-discovered downstream target genes of p53 capable of inducing apoptosis. We are interested in extending these results in apoptosis to other wild-type p53 expressing cells and to other cellular phenotypes such as neuronal cells to determine possible roles in neurodegeneration and cell death.